Printers, such as ink-jet printers and laser printers, are utilizing increasingly higher resolutions to provide higher quality output. Typically, increasing the resolution of a printer results in substantially increasing the cost of the printer. One way to keep the cost of a printer down while still increasing the resolution of the printer is resolution doubling. Resolution doubling allows a printer having a native resolution lower than the resolution of an image to be printed to print the image while maintaining the quality of the higher resolution image. For example, resolution doubling allows a 600 dpi printer to print a 1200 dpi image, or allows a 300 dpi printer to print a 600 dpi image. The higher resolution image is printed on the lower resolution printer without substantially increasing the cost of the printer. In addition, the output of the lower resolution printer provides substantially the same quality output as that which can be provided by a higher resolution printer.
Typically, resolution doubling occurs in hardware in the printer. The printer receives an image, such as a 1200 dpi, 1-bit per pixel image, and converts the image into a 1200 dpi horizontal by 600 dpi vertical, multi-bit per pixel image. Resolution doubling is typically performed by analyzing a four row by three column window of pixel data. The windows of pixel data are compared to a small number (16-32) of templates. When a match is found, the center pixel of the window is replaced by a single P-code for controlling the print engine that outputs the image to media. The P-code controls the modulation of the laser in a laser printer during a single pixel period. The P-code represents a single pulse width value along with placement information (left, right, center, or split). In part, resolution doubling is accomplished in the horizontal direction by providing p-codes at twice the print engine's normal rate. In part, resolution doubling is accomplished in the vertical direction by modulating the laser such that two vertically adjacent pixels do not receive sufficient charge to attract toner by themselves, but the overlapping region between the two pixels does receive sufficient combined charge to attract toner.
In the horizontal direction, resolution doubling is typically accomplished by keeping the horizontal resolution at 1200 dpi by increasing the modulation of the laser in the horizontal direction. For a 1200 dpi 8.5 inch wide image using 8-bit P-codes, the resolution doubling hardware produces 10,200 8-bit P-codes for a total of 81,600 bits per pair of input rows. Processing of all these bits in hardware increases the cost of the printer.